An Ion Sensitive Field Effect Transistor (ISFET) device is typically implemented in a pH sensor used for measuring ion concentrations in solution. When the ion concentration (such as H+) in the solution changes, the current through the ISFET device will change accordingly. The solution is used as the gate electrode of the transistor formed by the ISFET device.
In the environment of deep sea pH sensing, large cyclic pressure changes over depth can lead to deformation and creep of pH sensor packaging that includes an ISFET sensor die. This mechanical change of the pH sensor packaging can alter the stresses on the sensor die and changes its resistance due to piezoresistance effects. These changes are not readily predictable and cannot be compensated for in the initial calibration of the pH sensor in the factory. In addition, changes in the pH sensor are typically not directly observable over time, which can lead to poor performance over the lifetime of the pH sensor.